Usually, ultrasonic waves are collectively referred to as sound waves of at least 16,000 Hz and can inspect the interior nondestructively and harmlessly, having thereby been applied to various fields such as defect inspection and disease diagnosis. One of these is an ultrasonic diagnostic system in which the interior of a tested subject is scanned with an ultrasonic wave, and then based on a received signal generated from a reflective wave (echo) of the ultrasonic wave from the interior of the tested subject, an image of the interior state in the tested subject is formed. In such an ultrasonic diagnostic system, an ultrasonic probe to transmit and receive an ultrasonic wave with respect to a tested subject is used. As this ultrasonic probe, an ultrasonic transmitting and receiving element constituted of a oscillator is used in which an ultrasonic wave is generated via mechanical vibration based on a transmitting signal, and a received signal is generated by receiving a reflective signal of the ultrasonic wave generated based on the difference in acoustic impedance within a tested subject.
In recent years, a harmonic imaging technology has been studied and developed to form an image of the interior state within a tested subject, not based on a frequency (basic frequency) component of an ultrasonic wave having been transmitted into the tested subject interior from an ultrasonic probe, but based on its harmonic frequency component. Such a harmonic imaging technology has various advantages as follows: (1) the sidelobe level is smaller than the level of a basic frequency component and the S/N ratio (signal to noise ratio) is improved, whereby contrast resolution is enhanced; (2) higher frequency is realized and then beam width becomes narrowed, whereby lateral resolution is enhanced; (3) in a close range, sound pressure is small and also sound pressure variation is minimal, whereby multiple reflection is inhibited; and (4) the attenuation beyond the focus is comparable to that of a basic wave and a larger deep velocity is realized compared with the case of use of a high frequency as the basic wave. For an ultrasonic probe used in such harmonic imaging, a broad frequency band is required ranging from the frequency of a basic wave to the frequency of a harmonic. The frequency range of the low frequency side is used for transmission to transmit the basic wave. In contrast, the frequency range of the high frequency side is used for reception to receive the harmonic (for example, refer to Patent Document 1).
The ultrasonic probe disclosed in Patent Document 1 is an ultrasonic probe which is applied to a tested subject to transmit ultrasonic waves into the tested subject and to receive the ultrasonic waves having been returned via reflection within the tested subject. This ultrasonic probe has a first piezoelectric layer containing a plurality of arranged first piezoelectric elements with a predetermined first acoustic impedance to transmit a basic wave having ultrasonic waves of a predetermined central frequency toward the interior of a tested subject and to receive the basic wave among the ultrasonic waves having been returned via reflection within the tested subject, and further has a second piezoelectric layer containing a plurality of arranged piezoelectric elements with a second acoustic impedance, which is smaller than the first acoustic impedance, to receive a harmonic among the ultrasonic waves having been returned via reflection within the tested subject. Herein, the second piezoelectric layer is entirely layered on the first piezoelectric layer on the side in which this ultrasonic probe is applied to the tested subject. Therefore, the ultrasonic probe can transmit and receive ultrasonic waves in a broad frequency band with such a constitution. For a basic wave in harmonic imaging, a sound wave having as narrow a band width as possible is preferable. As a piezoelectric body playing such a role, a single crystal such as crystal, LiNbO3, LiTaO3, or KNbO3; a thin film such as ZnO or AlN; and a so-called inorganic piezoelectric material obtained by polarization treatment of a fired body such as a Pb(Zr,Ti)O3 based body are widely used. These piezoelectric materials of inorganic materials have features such as high elasticity stiffness and mechanical loss coefficient, as well as high density and dielectric constant. On the other hand, for a piezoelectric element to detect received waves of the high frequency side, sensitivity is required in a broader band width. Therefore, these inorganic materials are unsuitable.
As a piezoelectric element suitable in the high frequency and broadband range, an organic piezoelectric material employing an organic polymer substance is known. There have been developed organic piezoelectric materials such as, for example, polyvinylidene fluoride (hereinafter referred to as “PVDF”), polyvinylidene cyanide (hereinafter referred to as “PVDCN”), and a polyurea resin containing a ureine group obtained from a diisocyanate compound such as 4,4′-diphenylmethane diisocyanate (MCI) and a diamine compound such as 4,4′-diaminodiphenylmethane (MDA) (refer to Patent Documents 2-4). These organic piezoelectric materials exhibit excellent processability such as thinner layer formation and larger area formation, being able to produce any appropriate shape and configuration. These materials have features such as small elastic modulus and dielectric constant, producing whereby features enabling high sensitivity detection in view of use as a sensor.
However, when an ultrasonic probe is formed using any of these organic piezoelectric materials, piezoelectric characteristics are inadequate, and especially in high temperatures, its physical properties such as piezoelectric characteristics and elasticity stiffness tend to decrease to a large extent. Therefore, there have been noted problems such that the applicable temperature range is limited; and piezoelectricity is impaired and deformation is produced by heating during production.    Patent Document 1: Unexamined Japanese Patent Application Publication (hereinafter referred to as JP-A) No. 11-276478    Patent Document 2: JP-A No. 6-216422    Patent Document 3: JP-A No. 2-284485    Patent Document 4: JP-A No. 5-311399